This invention relates to test structures used in semiconductor manufacturing, and in particular to test structures used to determine the print quality and resolution of in a lithographic process.
Photomasks are an integral component in the lithographic process of semiconductor manufacturing. Semiconductor manufacturers use photomasks to optically transfer (e.g., print) images of devices (e.g., integrated circuits) onto semiconductor wafers. A lithography tool called stepper projects light through the photomask to print the image of one or more devices onto a field on a silicon wafer coated with photoresist. The stepper then moves (e.g., steps) the wafer and the image is exposed once again onto another field on the wafer. This process is repeated for the entire wafer surface. When using a positive photoresist, the exposed portions of the photoresist are removed so areas of the wafer underneath can either be etched to form channels or be deposited with other materials. This process can be reversed using a negative photoresist where the unexposed portions of the photoresist are removed.
In lithography, the print resolution is critical. Print resolution is the smallest line and space that can be precisely printed (matching the mask dimension.) Print resolution depends predominately on the focus of the stepper, but could also depend on various other factors. For example, if the stepper is not able to print one micron at a particular time, then the stepper cannot be used to produce one micron devices until maintenance and repair are performed.
Thus, what is needed is an apparatus and method for processing engineers to detect stepper problems before they affect the production runs.
In one embodiment of the invention, a test structure pattern includes a first comb, a second comb, and a serpentine line. The first comb includes a first set of tines of the same orientation. The second comb includes a second set of tines of the same orientation that are interdigitated with the first set of tines. The serpentine line runs between the interdigitated tines of the first comb and the second comb. The test structure pattern forms a first metal comb, a second metal comb, and a serpentine metal line on a die.
In one embodiment, electrical continuity is checked on the serpentine metal line. If a current cannot flow through in the serpentine metal line, then the serpentine metal line is broken or discontinuous. This suggests the resolution of the stepper is compromised due to being overexposed or out of focus.
In another embodiment, electrical continuity is also checked between the serpentine metal line and at least the first metal comb and/or the second metal comb. If a current can flow between the serpentine metal line and either the first metal comb or the second metal comb, then this implies there is bridging across a gap where there should not be any conductance. This suggests the resolution of the stepper is compromised due to being underexposed or out of focus.